Exercise 20
To study turbidity of water samples
To study turbidity of water samples
Principle:
Various characters that control the quality of water are taste, smell, colour, amount of dissolved nutrients, dissolved O2 and CO2, pH and different types of plants and animals and their density. Turbidity of the water body determines the depth upto which light can penetrate and thus affects the distribution and photosynthesis of phytoplanktons and macrophytes. More turbid the water body less is the thickness of its photic zone.
In polluted water bodies turbidity is due to:
Effluents: A water body which receives domestic sewage, run off from adjacent agricultural fields and liquid wastes from nearby small and large industries remains turbid.
Planktons: A water body may be turbid due to very high density of phytoplanktons and zooplanktons, especially when the water body is rich in nutrients.
I. Secchi’s Disc method
Requirement: Secchi’s Disc, rope of moderate thickness, meter rod, black and white paints; paint brush. Prepare a Secchi's disc by taking an iron disc of about 6 inches diameter, to which a weight is attached in the centre on one side and an iron hook on the other side. Tie a plastic rope of sufficient length to the hook. Divide the upper surface of the disc into 4 equal segments and paint two of these white and the other two segments black in such a way that black and white segments alternate with each other (Fig. 20.1).
Procedure:
(i) Visit a nearby pond.
(ii) Reach to the center of the pond in a small boat.
(iii) Slowly immerse the Secchi disc into water vertically holding the rope tightly in the hand till the black and white segments of the disc just begin to disappear. On reaching to a particular depth, the disc becomes completely invisible. Mark the length of the rope when the disc just disappears
(say A cm).
(iv) Slowly pull up the disc and find out the length of the rope where the black and white segments of the disc just reappear (say B cm).
(v) Find out the mean length (X) of the rope by the following method.
X=A+B/2 cm
(vi) Repeat the process at different sites of the pond.
Observations:
The value X represents the depth of the photic zone upto which sunlight penetrates in the water body and photosynthesis takes place.
Discussion:
Greater the value of 'X' less turbid is the water. In crystal clear deep lakes, the value of 'X' will be very high indicating, thereby, that the water body does not have large quantities of flocculating silt or organic matter residues. This may be due to no discharge of effluents or domestic sewage into the water body. The high clarity of water is also an indication of very less density of phyto and zooplanktons. These water bodies are called as non-productive or oligotrophic, while highly turbid water bodies are eutrophic in nature.
Precautions:
Students are advised to perform this experiment under the strict supervision of teacher to prevent incidents due to drowning.
II. Measurement of turbidity using measuring cylinder
Requirements: Water samples from different sources, three measuring cylinders (500mL) of the same height.
Procedure:
(i) Collect about 2 liters each water samples from different sources.
(ii) Transfer 500ml of water sample in the measuring cylinders of same volume and height.
(iii) Mark the three cylinders A, B and C and leave them undisturbed overnight.
Observations:
Observe the amount of sediment settled at the bottom of each cylinder and also note whether the water above the sediment is still turbid.
Record your observations in the following table:
Discussion:
Do all the samples show same amount of sediments?
Which sample shows maximum sedimentation and correlate it with the source of the sample?
Find out whether in all the cylinders, water above the sediment is clear or turbid. Explain with reasons.
Draw conclusions on the basis of the observations.
Questions:
Is turbid water fit for drinking? Explain.
Why is the penetration of sunlight in any water body important?
Green plants are seen only in photic zone. Comment.
It is a common practice to use alum for clearing turbid waters. Explain.
Turbidity of water body varies with season. Comment.